1. Field of the Invention
The invention relates to a dimming device, and more particularly to a dimming device for adjusting color temperature and color rendition index (CRI).
2. Description of the Related Art
Dimming methods for a light emitting diode (LED) include an analog dimming method and a digital dimming method.
According to the analog dimming method, a voltage across the LED is adjusted to non-linearly raise or reduce a current flowing through the LED such that the brightness of the LED is adjusted to achieve different lighting effects. However, since variation of an amount of the current may result in the wavelength drift of light emitted from the LED, color temperature cannot be exactly controlled.
According to the digital dimming method, for example a pulse width modulation (PWM) dimming method, the LED is driven by a PWM signal with a fixed amplitude and various duty cycles. In other words, the brightness of the LED is adjusted by controlling a ratio of on-off time of the LED, i.e., the duty ratio of the LED, in response to the PWM signal. In this case, due to the persistence of vision, light emitted from the LED has a relatively stable wavelength, thereby facilitating control of color temperature as compared to the analog dimming method. However, for a white light LED formed by means of a blue light LED coated with a yellow phosphor, although such a white light LED is driven by a PWM signal, desired color temperature may not be achieved.